Factual error: While the tidal wave is enveloping Manhattan, there is a scene where Washington Square Park is destroyed. A large arch is hit by the wave before reaching the rest of the park. The problem is the arch is on the UPTOWN side of the park, not the DOWNTOWN side which according to the movie was the direction the wave was coming from.
Factual error: When Sarah's family is fleeing Richmond, the highway sign says "Interstate 66." That highway is about a hundred miles north of Richmond; it starts in Washington DC. Another highway sign says "Virginia Beach 12 miles", which is the wrong way when fleeing Richmond westward.
Factual error: After the initial impact of the small comet, the clouds are all blown away. Yet in every further scene there is normal cloud cover, including the shots from space.
Factual error: When the asteroid crashes on earth you see people looking up at it, to see it fall. In reality the asteroid would burn everything on the ground due to the heat it develops.
Factual error: Leelee and Elijah are supposed to live in Richmond, Virginia. However, the hill supposedly behind Elijah's house is too high and too open for the Virginia piedmont and clearly looks like California. (Similarly, the hills around the Ark project, supposedly located in the Missouri Ozarks, also look very Californian.) When the couple are escaping the tidal wave on the motorcycle through the woods in the mountains to the west, they are going through a very open forest of what appears to be some kind of western pine trees found in semi-arid areas. In reality, the underbrush in the moist Virginia Blue Ridge would be too thick to drive through outside of a road or wide trail.
Factual error: Otis Hefter is introduced as the Director of the mission and prior to the launch of the Space Shuttle, greets someone with, "Welcome to Houston!" Hefter is then portrayed as the person "in command" of the mission, including the countdown and lift off. This is incorrect as Ground Control at Cape Canaveral, Florida commands the mission until the spacecraft has cleared the launch tower and actually begins its flight and mission. It is only then that the command of the mission who be handed over to Hefter and the crew in Houston.
Factual error: Many scenes showing the tidal wave approaching land and buildings is factually inaccurate. Its already stated that the tidal wave will be moving faster than the speed of sound, and reach a height of over 1000 ft by the time it hits land. That being the case, there would be a massive pressure wave of air in front of the tidal wave, that would clear a path long before the tidal wave hits. Buildings would start to topple and crumble before the water struck, and certainly people would not be able to stand in the path of the tidal wave, because the air pressure would blow them away.
Factual error: When the comet is passing through the atmosphere it shines about as bright as your average bike-light, and the shot shows thousands of people looking directly at it without shutting their eyes. When a comet of that size goes through the atmosphere it does so at an incredible speed. The friction between the air molecules and the comet produces a light many times brighter than the sun, anyone looking at it would be blinded instantly.
Factual error: The shock wave that hit Messiah when it detonated the nuclear bomb on the comet would not have existed in space. It may have been hit by debris, but that would have destroyed the craft.
Factual error: A wave 1000' high would have obliterated the containments and wiped out the spent fuel pools of every nuclear plant on the east coast. Even if they were all shut down, there would be no time or way to remove all the fuel and no place to safely store it all if they did. There are thousands of metric tons of fuel that have to be actively cooled for years, otherwise it would all catch fire and burn, or at the very least boiled all its cooling water away and melted/uncovered it to allow gamma shine. This would have, in and of itself, made the northern hemisphere uninhabitable for several hundred human lifetimes.